1. Field of the Invention
This invention relates to a system for creating computer graphic models and rapidly displaying the models.
2. Description of Related Art
Systems have been described which start with a block of volumetric data such as that acquired from a medical imaging system, determines interfaces between adjacent data values indicating a change in the measured value, and then models the surfaces with triangular elements having a vector normal to the surface at each of the vertices of the triangle. This is described in the "marching cubes" patent, U.S. Pat. No. 4,710,876 "System And Method For The Display Of Surface Structures Contained Within The Interior Region Of A Solid Body" by Harvey E. Cline and William E. Lorensen, issued Dece. 1, 1987. Thresholds are used to determine significant changes between adjacent values in the volumetric data which indicate movement across surfaces. These surfaces are saved as a computer model which may be displayed at some later time when a viewing angle is selected. The surfaces are shaded relative to a viewpoint and therefore are dependent upon the viewpoint chosen.
This model provides perspective views of the model data and provides realistic images of the surfaces. It, however, is computationally intensive to convert the lists of triangles and normal vectors into surfaces during display time.
Another method as described in the "dividing cubes" patent, U.S. Pat. 4,719,685 "Dividing Cubes System And Method For The Display Of Surface Structures Contained Within The Interior Region Of A Solid Body" by Harvey E. Cline, William E. Lorensen and Siegwalt Ludke, issued Jan. 12, 1988, describes another method of displaying volumetric data. Again, changes between adjacent data values are monitored in order to find changes above a specific threshold which indicate an interface in the data. All interface points throughout the data block are identified, then converted to a number of points with the pixel coarseness of an image desired to be displayed. A normal vector for each of the surface points on the model is determined and associated with each point. Each point, its location and three-dimensional space, and its normal vector are stored as a point/normal model to be displayed during display time.
During display time a viewpoint is determined and the component of each normal vector pointing towards the viewpoint is used to shade each of the points of the surface to result in an image. This image may be rapidly displayed once a viewpoint is selected, however due to the nature in which the model is created, the coarseness of the model is the same at points near an image plane as well as that far from the image plane. This results in an image having no perspective.
It is important to note that it is very desirable to have a computer model which is transportable, that is to say, may be displayed easily on one type of computer graphic display, or moved to a different type of computer graphic display and also be displayed on that system. Being that the model of the marching cubes system is a "wire mesh" model and is incompatible with the system used to display the "point/normal" model as described in the dividing cubes patent, and vice versa. Therefore, the models would not be interchangeable, and would have to be generated from scratch for each of the two systems. Also it must be noted that CAD/CAM systems typically display models which are stored in the model format of the marching cubes system.
Therefore there is a need for a computer graphic system which produces a model which is transportable, and may be rapidly displayed.